a) Field of the Invention
The present invention relates to an integrated optical device, and, more particularly, to a hybrid integrated optical device having optical fibers, optical waveguides, and optical semiconductor devices integrated on a single substrate.
b) Description of the Related Art
A conventional integrated optical device will be described with reference to FIGS. 9 to 13.
FIG. 9 is a perspective view of a conventional optical IC disclosed in Japanese Patent Laid-open Publication No. 57-84189. Semiconductor laser guides (recesses) 58 are formed on a substrate 51, and semiconductor lasers 57 are fitted in the semiconductor laser guides 58 and fixed thereto.
Laser beams emitted from the three semiconductor lasers 57 are converged by a cylindrical lens 54 aligned in a lens guide groove 55 formed on the substrate 51, and guided to three light incident planes of an optical waveguide 52 formed on the substrate 51. The optical waveguide 52 guides the three beams, incident to the three light incident planes separately at first, and then multiplexes or joins them at the junction region thereof to output the same, as a composite laser beam, from a single output plane thereof.
Along the optical axis of the outputted laser beam, a V groove 56 for guiding an optical fiber 53 is formed on the substrate 51, starting from the position corresponding to the light output plane to one end of the substrate 51. The optical fiber 53 is fitted in, and fixed to, the V groove 56.
The positions of the semiconductor lasers 57 and optical fiber 53 are therefore aligned by the semiconductor laser guides 58 and V groove 56 to provide optical coupling therebetween.
FIGS. 10A to 10F illustrate two different procedures of forming the optical fiber guiding V groove 56 (hereinafter simply called a V groove) and the optical waveguide 52 of the integrated optical device shown in FIG. 9. In the procedure illustrated in FIGS. 10A to 10C, an optical waveguide is formed after a V groove has been formed.
As shown in FIG. 10A, an SiO.sub.2 film 62 is formed on the surface of a silicon substrate 61 having the (100) plane, and an opening 63 to be used for V groove etching is formed in the SiO.sub.2 film 62 by photolithography. By using the SiO.sub.2 film 62 as a mask, a V groove 64 is formed by anisotropic etching by using potassium hydroxide (KOH) aqueous solution, as shown in FIG. 10A.
As shown in FIG. 10B, a lower cladding layer 65 is formed on the whole surface of the SiO.sub.2 film. Next, an core region 66 having a relatively high refractive index is formed on the lower cladding layer 65 and patterned. An upper cladding layer 67 is then formed on the core region 66. As shown in FIG. 10C, the lower cladding layer 65, core region 66, and upper cladding layer 67 are selectively etched to form an end plane of the optical waveguide. This end plane corresponds to a light input/output port of the Planar wave guide.
The procedure illustrated 10D to 10F forms an optical waveguide before a V groove is Formed. As shown in FIG. 10D, a lower cladding layer 65, an optical waveguide core layer 66, and upper cladding layer 67 are formed on a silicon substrate 61 in the manner similar to the step of FIG. 10B.
Next, as shown in FIG. 10E, the lower cladding layer 65, core region. 66, and upper cladding layer 67 are selectively etched to form an opening 68 to be used for V groove etching. As shown in FIG. 10F, by using the optical waveguide components 65, 66, and 67 as a mask, the silicon substrate 61 is anisotropically etched to form a V groove 64.
With the method of forming an optical waveguide after a V groove has been formed, materials of the optical waveguide components are also deposited on the V groove 64 while the optical waveguide formed. It is difficult to remove such materials on the V groove while maintaining the design configurations off the V groove and optical waveguide, because the V groove is as deep as 100 .mu.m usually.
With the method of forming an optical waveguide before a V groove is formed, the optical waveguide films 65, 66, and 67 are side-etched when the V groove etching opening 68 is formed, because the optical waveguide is thick. It is therefore difficult to form the V groove 64 precisely.
Further, as seen in FIG. 9, the semiconductor laser 57 is fitted in the semiconductor laser guide 58 and bonded thereto in a junction-up position (with a heterojunction located upside). However, spatial allowance for the height of the active region of a semiconductor laser is above (i.e., greater than) +/-10 .mu.m, so that it is difficult to make the core region of a waveguide, to be coupled thereto flush with the active region.
Furthermore, the width of a semiconductor laser chip and the relative positions of the active region from opposite side surfaces have a dimensional allowance of above +/-10 .mu.m. Whereas, the position displacement less than +/-3 .mu.m is usually required for single-mode waveguide.
Therefore, the practical position displacement is large compared to the required precision if simply a semiconductor laser is fitted in the semiconductor laser guide 58, so that it is difficult to obtain sufficient coupling with an optical waveguide.
Still further, in forming bonding pads on the bottom wall of the semiconductor laser guide 58 by photolithography for bonding the semiconductor laser 57 thereto, the exposed image of a pattern has blurred edges because of the steps of the optical waveguide and the laser guide groove, so that it is difficult to provide precise position alignment. Yet still further, the V groove 56, which is very deep for photolithographic process, prevents fine work.
FIG. 11 shows an example of an integrated optical device disclosed in Japanese Patent Laid-open Publication No. 61-46911. As shown, an optical fiber guide 72, an optical waveguide 73, and semiconductor laser guides 74 are formed on a substrate 71.
A semiconductor laser 75 having an active region 75a and a photodetector 76 are aligned with the semiconductor laser guides 74 and fixed thereto.
In the example shown in FIG. 11, if a usual optical fiber 77 having a clad diameter of 125 .mu.m is used, it is necessary to set the height of the center of the core of the optical waveguide to 62.5 .mu.m, and the height of the optical fiber guide 72 becomes obviously higher than 62.5 .mu.m.
Therefore, if an SiO.sub.2 optical waveguide of high reliability is to be formed, it takes a long time to deposit the optical waveguide materials and etch and pattern the optical waveguide structure and guide structure.
Like the example shown in FIG. 9, the dimension allowance of the height of the active layer of a semiconductor laser and edge the positions of active region from its opposite side edges is above +/-10 .mu., so that it is difficult to make the active region 75a of the semiconductor laser 75 flush with the core region of the optical waveguide 73. The position alignment is particularly difficult if a single mode fiber and a single mode optical waveguide, which usually requires accurate positioning less than +/-3 mn, are used.
FIG. 12 shows an example off an integrated optical device disclosed in Japanese Patent Laid-open Publication No. 2-58005. As shown, an optical waveguide 87, and bonding pads 81, 82, and 83 are formed on a substrate 80.
A bonding pad 86 is formed on the bottom of a semiconductor laser 84 and bonded to the bonding pad 82 on the substrate 80. In this case, the semiconductor laser 84 can be position-aligned precisely because, if there is any misalignment, the bonding pad 86 makes a short circuit between the bonding pad 82 and the bonding pad 81 or 83.
However, a fine pattern of the bonding pads 81, 82, and 83 is difficult to be formed by photolithography because there is a step of the optical waveguide 87. It is therefore difficult to precisely align the active layer (or region) 85 of the semiconductor laser 84 with the core of the optical waveguide 87.
As described above, precise optical coupling between a semiconductor laser and an optical waveguide or between an optical waveguide and an optical fiber is difficult because the pattern formed by photolithography has blurred edges because of steps of the optical waveguide and the like or because of the dimension allowance of the active layer of the semiconductor laser.
If materials of an optical waveguide enter a V groove, it is difficult to remove such materials while maintaining the designed configuration of the V groove.